Making a foam cored wing using 2-part rigid polyurethane foam in the mould.

I’ve been playing around with putting 2 part polyurethane foam inside composite skins which have been vac bagged and cured in the molds, then closing the mould so the foam expands and fills the skins to give a foam cored wing.

I’ve been doing reports about it on one of our local threads, and was urged to post the results in this composites fabrication. I’ve been reluctant to do so because it’s just backyard, rough and ready, trial and error stuff and I know there are a lot of very experienced guys who read this forum. And I haven’t researched the forum to see what others have done already. But I’ve had some success which has prompted me to start this thread.

I’m basically a slope nut, and when I got my first “moldie”, an Aeromod Minij and later a Miraj, it was obvious that the foam cores conferred huge durability compared to hollow molded planes. But other than Aeromod, who don’t even have a dealer in Australia now, there are no other manufacturers who make foam cored planes. One or 2 others tried and gave up soon. So before long I got some lessons in vac bagging and made foam cored wings that way. Most of those wings are still flying, either with me or my mates, and have outlived numerous hollow moldies.

Just over a year ago I designed a simple plane I called the Scratcho, which has a 64” span, HN1038 foil (i.e. thin!) fairly high aspect ratio wing. I shaped a fuse and made a mould of that and vac bagged glassed tails from flat balsa. Basically the Scratcho flies fabulously – fast and aerobatic and locks beautifully into a DS circuit. The vac bagged one did 185mph on the first day’s DSing, but more than that was just all round great fun and totally free of bad habits. So that enthused me to make a plug and from that, a wing mould. I had never made, owned or used a wing mould before so it was a great learning curve. The result isn’t perfect, but good.

My total goal was to make molded wings with foam cores. In that quest I talked a lot with Timbuck, who was on the same wavelength. At the time he had discovered Ampreg F230 foaming epoxy, and he was very excited about it. One Sunday we had a crew of blokes turn up here at 8am. We cut the glass, mixed resin with fast hardener and laid it up, added spar caps etc and vac bagged it with heat for a few hours. Then we unwrapped it, added a few bits of blue foam in servo spots etc, then mixed 600mL of Ampreg and poured it into the skins, smeared it around, closed the mould and ramped up the heat. At 8pm the same day, there was a bunch of guys leaping around for joy as we broke the mould open and pulled out an incredibly strong wing, filled solid with Ampreg epoxy foam. We called it the “Timbuckerized Wing”.

This was just fantastic. We made another, improved one soon and I owe a lot of my DS learning to the durability of the Ampreg F230 filled wing. But the stuff is quite dense, so wings filled with it are at least 1.3kg, and need reasonable air to perform well. So I began to work on ways to get the benefits of the F230 without the weight, by putting pre-cut blue foam cores into the wing skins before pouring in Ampreg. Several prototypes later I’d made some great slope wings, but they weren’t ideal … I’d found that the Ampreg doesn’t like to expand when used in a thin gap. It wouldn’t completely fill and bond between the foam core and the skin. It seems to need a critical thickness or mass to activate the expansion.

I then cut half –foil foam cores … i.e each core had just the top or bottom half of the airfoil cut out of a thin foam block. They were vac bagged into the molded cured skins, (with some epoxy painted into the skins). Once the foam had bonded to the skins I hot wire cut the excess foam off, flush with the mould flanges so there were 2 fully foam filled halves, ready to mate together. I cut slots for a socked spar and socked aileron stiffeners, and also cut the foam away from the edges of the entire circumference of the wing, to allow some space to pour a thick bead of Ampreg F230 all around. Then the mould was closed. The end result is, to me, perfect. Acceptably light (540gm wing out of mould) and extremely strong. The Ampreg around the edges makes the wing so tough that it’s no problem to crash land the plane into the bushes where there’s no LZ. It’s so good, I wish all slope planes were so tough. But it was quite a lot of work.

Recently, Two-Stroke in Slovenia reported using 2-part polyurethane foam inside a new plank wing. I had ditched the idea of PU foams for filling. Timbuck had played with them and made some Warhead 30 wings by putting foam into the skins and allowing both sides to expand … then he cut the excess off with a knife or saw and bonded them together. That sounded hard to me; I wanted to have the foam expand inside the closed mould, but Tim said the working time was impossibly fast. I asked Two Stroke how he did it … he had done it in 2 steps, part filling with PU the first time, then trimming and putting PU in again and quickly closing the moulds. So this got me interested again.

About a month ago, I had obtained some PU foam called Erapol GP330 which is a commonly available, cheap insulating/ floatation foam. I tested it and was amazed at the expansion. But the stuff goes off fast. You have about 10 seconds mixing time, then about 30 seconds to spread it as it begins to froth but once it starts to gel, you can’t touch it or the matrix will collapse at slight touch. I was worried about the short working time but found that all the other foams were even faster!

I worked out the volumes of my fin and stab moulds, and vac bagged light glass/ carbon skins into them. When they were cured, one at a time I mixed the PU (1/12th of the volume) and smeared it into the skins (both sides) and closed and clamped the moulds. I really didn’t expect it to work, but when I opened them up the next day I was honestly stunned by how good the parts were. Much, much lighter than I’d made before, and very stiff. Even after cutting the control surface hingelines, they were good and strong. This motivated me to proceed with trying it on a whole wing!!

A few days ago I laid up light glass/ carbon skins in my Scratcho wing moulds and vac bagged them down. When the skins were cured I used fine sand leveled in the moulds to calculate the total volume. Yesterday I got a helper and mixed the foam and spread it as a long thick band along each half. We were very prepared and worked fast, but this stuff goes off fast. We had spent too long mixing, and when I started trying to spread and even up the liquid, it was already starting to collapse when touched. So we closed the mold. A few hours later I opened it up, and was very disappointed to find that it was a failure. The foam had not expanded as far at the LE and TE from the central band. So there was a big void behind the LE and TE joins.

This was puzzling, because small amounts of foam such as the leftovers in the mixing cups expand hugely. I had been advised that the foam didn’t like to expand far from a pool of liquid. But I didn’t realize that the problem would be this bad! It seemed that when you start with a lot of liquid in one spot, it starts to expand but the surface tension at the outer edges increases, resisting expansion from the centre and it gradually stops moving, not reaching the potential expansion. This results in a lot of foam of high density, and gaps at the edges. Thinking about this I wondered if spreading the foam liquid more evenly, and coating the entire skins, would allow it to expand more fully and fill the wing better.

I was kicking myself for wasting good materials making nice carbon skins for a failed experiment, then had an idea … and with very little effort I found that it’s not hard to peel the skins back off the rigid foam! So I was able to salvage the skins in perfect shape, scrape almost all the PU foam off them, and try again. To get the skins to sit back into the moulds, I misted 3M 77 glue spray over the skins and mould, which sounds bad but is important because you have to make sure the expanding foams can’t bleed back into a gap between the mould and the outer skin surface. With the 77 spray, they stuck down nicely.

At lunchtime today we did “Take Two” and this time we did things a bit differently:
1) We timed the mixing to only 10 seconds, then quickly poured into both skins.
2) As soon as it was poured, 3 sets of hands set about spreading the mixed GP330 across the whole skin area from LE to TE, very quickly. Even so, we were still working on the spreading when the foam began to collapse to the touch.
3) We used 200mL total liquid this time (1/10th volume) instead of 160mL (1/12th).

I opened it up this evening, and it’s a 97% success! You would find it hard to credit the difference in the result compared to yesterday’s experiment. Basically it's perfect except for 2 quite small sections of the leading edge that have a bit of a cavity behind them. But they're pretty small deficits. I can tap all along the LE and it's solid. The TE is bonded all along, and feels really solid and rigid too, apart from one small spot that I can bend. The wing is definitely stiff both in bend and torsion. Weight is 460gm, which is 40gm more than yesterday's because we used 40mL more GP330!

Sorry for the long rave! In summary so far, I think the method does have potential if you're prepared to set everything up ready and have a few guys to mix and spread it VERY quickly. I'm going to continue with trying it for different things. I reckon you could vac bag a glass and Innegra skin overnight, and next day get 4 guys together and spend 10 minutes doing the foam mix pour and spread at sprint speed, and 3 hours later you can pull out a brilliant very durable cheap wing.

Here’s a short video I took tonight showing the results and explaining a bit about it.

I did make one wing with two components foam in the past.
With same results /problems as You showed in the video.
First and foremost was skin de lamination.
Unfortunately,after sitting in the shop shelf for a while, wing went to the garbage can and
I did not bother with the pictures.
If I will do it again,I would work with polyester resin,not epoxy to make wing skins.
Also ,I would not wait until skins will be completely dry ,starting "foaming" work part on tacky surface.
It is doable ,just needs technology to be "polished".
Yuri.

* One large UAV I have seen used a similar approach but the cores were made with a spacer( thick, release coated mylar) in the mold and the foam was put into the mold and then expanded. After the core was cured, the spacers were replaced with woven carbon and the part was infused.

* you may consider a dedicated mold for making the foam core. Why? this mold I saw had vents and ports at key points to let the material foam up into the vents( which are later machined off). this prevented poor density in key areas due to trapped gasses and flow problems.

* In this application, a conduit for wiring, and a leading edge reinforcement were added in the mold prior to pouring in the foaming urethane. This made it easy to finish the wing and increased its durability.

* the part was post-cured to 175F I believe to harden the foam and increase compression strength.

This part was 25 ft. long x 12 ft. wide and the mix and pour was with a 55 gal. then a forklift then dropped the upper wing mold onto the lower one and clamps were quickly put in place. I saw this in 1997.

I have quite a bit of info and tips to post but need more time to gather it and write it all up. Stay tuned.... but to quickly note as Scott mentioned, you may be better off to make a specific mold for the cores. I have seen full scale propellors made this way and infused later with epoxy/glass/carbon.

Regarding the bonding of the PU foam to the skins ... in this case the PU had bonded very well. When I tore the skins away from the foam (after the first, failed attempt), a layer of PU was left adhered to the skins which could not be completely scraped off. The foam is brittle / friable, so all that happens is that the matrix breaks away a fraction of a mm from where it's bonded to the surface. I'll attach a photo.

Regarding making a separate mould ... or using a liner to create undersized cores ... I need to think about that! I guess if I was going to stick with a standard skin thickness etc it would work but we tend to make some light ones, and some with thicker skins for DSing etc. Plus experimenting with different spars etc. The nice thing about foam that expands in the mould is that it works its way around whatever hardware you have in there.

It would be great if you guys could put together any relevant info here. Doing a search, I did see some interesting posts - particularly yours Troy, so any help would be appreciated.

The Ampreg F230 filled wings are almost impossible to kill!!! I've had a plane go straight into the hill at massive speed, totally destroying the fuse and tails and the wing was fine! (but all busted servos).

I wonder, is there a mixing/dispensing system that isn't too expensive to try? If you could spray the foam into the mold you might have more control over the application and more time.

I have a buddy that sprays two-part PU foam into wall cavities for a living, he knows EVERYTHING about this stuff. The expense of spray equipment is in it's ability to exactly meter the flow rate of the chemicals. Given the expansion ratio, measuring this stuff precisely while it's flowing to a spray gun is tricky business. The stuff he sprays has a sub-second mix time and goes off in two-seconds flat. Even so, properly metering the slower stuff would still be extremely challenging.

As you've discovered, the trick to getting it to stick well is to smear it around onto all surfaces before it goes off. When it goes, it forms a skin on any non-contacting surfaces. The skin doesn't stick well to anything, and you do have to be careful to provide venting forward of propigation. I suspect what's happening here (in your favor) is your allowing some gasses to escape through the perimeter opening (which is good).

Though it's likely you've got some voids in the center of the wing where the expanding (inward) skins met. This area will also not be very well stuck together, and be a much higher density then other areas. What my foam guru suggests is, do left/right wing-halfs, preferably open at BOTH ends if possible, and over-fill them by by some amount (you'll have to experiment).

The goal is to get the expansion going to such excess, that the inner-skins are forced out the end(s). You'll waste a lot of foam this way, however the result will be much more consistent density, and you'll have less problem with edge-voids (though smearing around thoroughly (like you are) is critically important.

I'm not sure about down-under, but here in the States they are coming out with slower-reacting low-density foams.. Granted it's probably a whole 10-20 seconds slower than yours, but every extra bit helps with this stuff. Good luck, and I'm glad you're brave enough to try this seriously, and really appreciate your sharing! Keep us updated!

Thanks Andwew, I have some foam at home that I put aside because I thought it would be too fast. I might just mix some up and see what it does. Does temperature affect the reaction speed?, cause I bet its colder down here in Victoria. I guess Ill find out....

To clarify a couple things, spray insulation foam and the two-part castable foams may be similar in that they are two part polyurethane based but they are very different in terms of physical properties and practical use (especially in this case). The insulation foams are typically 1.5lb/cu" density (24kg/cubic meter) or less and quite spongy/brittle. The equipment used is typically a high pressure impingement spray gun that forces pre-heated A&B together just in front of the gun so it mixes in the air. It's ridiculously fast stuff. The equipment for two part castable foams is lower pressure but uses a mix tube for proper blending. You can hand mix or machine dispense castable PU. You cannot realistically hand mix spray foams.

The key thing to note when comparing the GP330 (PU based) to the Ampreg (epoxy based) used by Timbuck is density. We are talking about a 2lb foam vs a 12 pound foam. This presents a huge difference in physical properties in the cured foam. Also, the lower density PU castable foams will begin to "cream" or react faster than a higher density castable PU foam. So, you get more working time with a 6lb foam than a 2lb foam. There are also "slow rise" formulations of castable PU foams but they do not typically work well when cast in thin sections. They are designed to do larger pours and control the exotherm in mass.

So, my suggestion would be to look at either a 4 or 6lb density foam (64-96kg/cubic meter) to get a little more working time plus some added strength. I have seen 10lb PU foam used as filler in hollow spots to allow a vac-bag pre-preg repair for aviation skins. It's very strong stuff.

Temperature does affect the reaction time because it is a thermoset system just like epoxies, polyesters, and other polyurethanes. But, you will not get the same expansion rate if you purposely cool down the material before mixing. You also raise the viscosity and potentially make it harder to mix (to a certain degree).

Also, expanding foams are tricky to use if you do not have the tool set up properly. Pouring it on the open molds and brushing it around is going to be tricky as well (you found out why) and will be hard to get consistent results. Typically close molds are the best way to control the density and promote consistent cell structure. If your mold has gaps and the foam can go wherever it wants, then it will not produce a high quality product. Ideally you want a mold that has carefully placed vents (and even the vent size is critical) while keeping the rest of the mold sealed. This often creates a lot of pressure on the tool and a flimsy mold won't hold up well to the pressures involved.

Wow that video is excellent! Very very well done and great info. It answered a few questions (eg the bit about side drag against the container, and the bits about orienting vent holes ... ) Thanks heaps Troy.

A lot of questions ran thru my mind while watching it.

1) The effect of humidity ... we get a lot of variation, from over 90% in summer to down to 23% in westerly winds. Hmm, I wonder whether that's a big problem? And if so, how to manage it?

2) Colouring of the foam sounds interesting ... a white foam colour under a glass skin would provide a good light coloured backing so that you could use less yellow/ white paint.

3) Most importantly, from your experience Troy, if you were doing this, how would you do it? Basically I want to be able to lay up the wing skins in the mould ... I guess it's best to cure them first ... add the various hardware, and then add foam to fill the thing and close the mould and have it fully expand. A few bubbles etc aren't going to be a big deal ... the foam is there to support the skins and to absorb impacts. Are there better types of foam? Are there foams with longer working times??? I'd really appreciate any advice. I guess it's possible that there are no PU foams that will provide the answers, and maybe I'll go back to vac bagging blue foam half-cores in place etc. But I'd like to check out all the options. I'm after simple methods that can be done in the back shed, that provide durability ... and a 98% result rather than perfection.